A transformer that is one of common devices used in a high-frequency integrated circuit (IC) for a wireless communication system is used to convert impedance or combine electric powers. FIG. 1 is a diagram of a conventional transmission line transformer, wherein a primary transmission line 110 and a secondary transmission line 120 are formed on a semiconductor substrate. Also, primary ports 130-1 and 130-2 are connected to each end of the primary transmission line 110 and secondary ports 140-1 and 140-2 are connected to each end of the secondary transmission line 120. Referring to a cross-sectional view 150 taken along a line A-A′ of FIG. 1, an insulator is disposed between the semiconductor substrate and the primary and secondary transmission lines 110 and 120.
FIG. 2 is a diagram for describing a principle of the conventional transmission line transformer of FIG. 1, wherein when a current is supplied to the primary transmission line 110 in a direction shown in FIG. 2, a current is induced to the secondary transmission line 120 in an opposite direction. Also, if the conventional transmission line transformer is ideal and thus lossless, a current strength in the secondary transmission line 120 is the same as a current strength in the primary transmission line 110.
In the conventional transmission line transformer of FIG. 1, highest layer metal lines available in semiconductor processes are used for the primary and secondary transmission lines 110 and 120, because when a distance between a metal line forming a transmission line transformer and a semiconductor substrate is decreased, a parasitic capacitance component is generated between the metal line and the semiconductor substrate, and thus a signal power loss is generated on the semiconductor substrate due to a magnetic field generated in the metal line.
Also, the current (hereinafter, a secondary current) of the secondary transmission line 120 is induced by the current (hereinafter, a primary current) of the primary transmission line 110 according to a magnetic field formed around the secondary transmission line 120 of the conventional transmission line transformer by the primary current. Generally, a coupling factor is used as an index indicating the strength of the secondary current induced by the primary current, and in order to increase the coupling factor, the magnetic field formed by the primary current needs to largely affect the secondary transmission line 120 of the conventional transmission line transformer. Accordingly, the areas of the primary and secondary transmission lines 110 and 120 facing each other need to be increased. However, the areas may be increased by increasing lengths of the primary and secondary transmission lines 110 and 120, but a power loss may be generated in the conventional transmission line transformer due to parasitic resistance components of the metal lines generated accordingly.
FIG. 3 is a transmission line transformer suggested instead of the conventional transmission line transformer. Referring to a cross-sectional view 350 of the transmission line transformer of FIG. 3, a primary transmission line 310 and a secondary transmission line 320 are disposed in same locations of different layers on a semiconductor substrate. Accordingly, primary ports 330-1 and 330-2 connected to the primary transmission line 310 and secondary ports 340-1 and 340-2 connected to the secondary transmission line 320 are disposed in same locations of different layers.
Parasitic resistance components that are same as those generated in the conventional transmission line transformer of FIG. 1 are generated in the transmission line transformer of FIG. 3, but a coupling factor is largely increased as areas of the primary and secondary transmission lines 310 and 320 facing each other are increased. However, at this time, a highest layer metal line and a lower layer metal line are used, and thus a distance between a metal line and a semiconductor substrate is decreased. Accordingly, a signal power loss caused by the semiconductor substrate is increased more than the conventional transmission line transformer of FIG. 1. Also, since the areas of the primary and secondary transmission lines 310 and 320 facing each other are largely increased, the transmission line transformer sensitively reacts to a signal change or a surrounding environment change.